The present invention relates to process transmitters used to measure process variables in industrial processing plants. More particularly, the present invention relates to a modular process transmitter comprised of interchangeable modules.
Industrial processing plants, such as chemical, pulp, petroleum, gas, pharmaceutical, food, and other processing plants use process transmitters to measure various process variables and communicate process variable information to a control system. These process variables can include pressure, temperature, flow, level, pH, conductivity, turbidity, density, concentration, chemical composition, and other properties of fluids. FIG. 1 is used to illustrate an example of a processing plant environment which includes various process transmitters. Process transmitters such as flow meter 2 in process fluid line 4, level transmitters 6 and 8 on tank 10, and a differential pressure transmitter 12 in process line 14, are shown electrically connected to control system 16 through a communication bus or control loop 18. Control system 16 is typically located in a remote process control room. Control system 16 can be configured to control, and receive process variable information from, the process transmitters over communication busses or control loops 18, in accordance with a communication protocol.
Process transmitters typically include a sensor portion having a sensor housing containing sensor circuitry, and a transmitter portion having a transmitter housing containing transmitter circuitry. The sensor portion is generally adapted to measure a process variable such as those mentioned above. The transmitter portion is generally configured to receive process variable information from the sensor circuitry and transmit the process variable information to a control system. The transmitter portion can also transmit and receive other information relating to, for example, the settings of the process transmitter. The process variable and other information is generally communicated in accordance with a standardized communication protocol.
Current process transmitter designs are not configured to allow the sensor portion to couple to add-on transmitter portions from other product lines or other manufactures. Such an ability could provide a way to enhance the capabilities of the process transmitter without the need for significant modifications. The primary reasons for this inability to xe2x80x9cmodularizexe2x80x9d a process transmitter is due to a lack of standardized features that would allow the sensor portion to be compatible with various transmitter portions and vice versa. Some of these non-standardized features include the manner of physically connecting the sensor portion to the transmitter portion, the seals used to protect the sensor and transmitter portions, and the software and electrical interfaces between the sensor and transmitter portions.
Additionally, current sensor portions cannot operate as stand-alone units that are capable of transmitting the process variable information to the control system. One reason for this is that the sensor housing is typically open to the inside of the transmitter housing to some extent to allow wires to pass through a threaded joint that provides a flameproof connection between the two housings. It would not be practical in a plant environment to run field wiring directly to the sensor housing without the use of a transmitter housing because the sensor housing, by itself, is not sealed and flameproofed from the environment. Additionally, the sensor circuitry, by itself, is not able to transmit information over a long distance.
It would be desirable to provide a process transmitter configuration where the sensor portion can operate in a field environment as a stand-alone process transmitter. Such a xe2x80x9cunitized sensor modulexe2x80x9d could measure process variables and communicate with the remotely located control system without the previously required transmitter portion. Furthermore, it would be desirable to configure the unitized sensor module to interface with add-on transmitter modules, from other product lines or other manufacturers, which expand the capabilities of the process transmitter. For example, the transmitter modules could expand the communications used by the transmitter and the functions it is capable of performing. When a transmitter module is coupled to the unitized sensor module, the unitized sensor module could operate as a sensor portion and the transmitter module could operate as a transmitter portion. However, this configuration could lead to problems dealing with xe2x80x9cnoisexe2x80x9d in the process transmitter.
Noise, produced by electromagnetic interference (EMI) and radio frequency interference (RFI), is often encountered in the design and operation of electronic equipment. EMI/RFI, as it is commonly known, can cause electronic equipment to function improperly or even fail. Process transmitters can pick up EMI/RFI from many different sources. One such source is the long wires of the control loop that connect the process transmitter to the process control system. These wires can act as antennas which receive radio signals thereby generating noise, in the form of high frequency current.
In order to reduce noise in the system, the transmitter circuitry includes EMI/RFI filtering circuitry that is generally adapted to create a bypass for the noise by forming a capacitive coupling between the transmitter circuitry and a circuit common, such as the housing of the process transmitter. This capacitive coupling provides a low-impedance path through which high frequency noise-related current is encouraged to pass prior to reaching the electronics of the process transmitter. The type of EMI/RFI filtering circuitry used by the process transmitter depends, at least in part, on the communication protocol the transmitter circuitry utilizes.
Since the unitized sensor module and the transmitter module each require EMI/RFI filtering circuitry, the use of dual noise filters might be contemplated when the unitized sensor module is coupled to a transmitter module. The existence of dual noise filters can present several problems. For instance, since the capacitive couplings associated with each of the noise filters are connected to circuit common, two separate paths exist for fault current to travel to the circuit common or ground. As a result, fault protection circuitry that is intended to protect the electronics of the process transmitter from fault conditions could be bypassed and, thus, rendered useless. This is a particularly unacceptable condition for process transmitters which often must meet strict intrinsic safety requirements due to the volatility of the environments in which they may operate. Additionally, dual EMI/RFI filters can create an undesirable ground loop, where an induced current passes from one grounding point to another grounding point, due to the dual connections to circuit common or ground. Furthermore, the existence of dual EMI/RFI filtering circuits could affect the performance of each, thus nullifying the particular EMI/RFI filtering solution being used by the process transmitter.
A modular process transmitter is provided that overcomes the problems described above. One aspect of the modular process transmitter of the present invention is directed to a sealed unitized sensor module that includes a sensor housing, a sensor circuit, a feedthrough, and a removable sensor EMI/RFI filtering circuit. The sensor housing includes a cavity and a fitting that is adapted to support a transmitter module. The sensor circuit is contained in the cavity and includes a sensor output that is indicative of a process variable and is produced in accordance with either a local format or a first communication protocol. The feedthrough seals the fitting and includes feedthrough conductors which are coupled to the sensor circuit. The removable sensor EMI/RFI filtering circuit is located externally to the sensor housing and is configured to create a bypass for noise in accordance with the first communication protocol.
Another aspect of the modular process transmitter of the present invention is directed to a transmitter module that includes a transmitter module housing, a communications circuit, and a transmitter module EMI/RFI filtering circuit. The transmitter module housing is mountable to the fitting of the sensor housing. The communications circuit is contained in the transmitter module housing and is couplable to the feedthrough conductors. The communications circuit is adapted to receive the sensor output from the sensor circuit, in the local format, through the feedthrough conductors and generate a scalable output in accordance with a second communication protocol. The transmitter module EMI/RFI filtering circuit is adapted to replace the sensor EMI/RFI filtering circuit and create a bypass for noise in accordance with the second communication protocol.